Existing mass spectrometers have an ion source that produces ions of a sample material. These ions are then processed by a mass analyzer which includes a mass detector. Some existing ion sources produce ions using a technique known as electron ionization (EI). Particles of a sample material that are referred to as analytes are supplied in a gas phase to an ion volume having a relatively low pressure, and an electron beam is also supplied to the ion volume. The electrons directly strike the sample analytes, and the resulting energy exchange is sufficient to cause ionization, producing ions characteristic of the sample material. These ions are then supplied to the mass analyzer.
A different type of ion source produces ions using a technique known as chemical ionization (CI). The analytes of the sample material are supplied in a gas phase to an ion volume, and a reagent gas such as methane is also supplied to the ion volume. Further, an electron beam is supplied to the ion volume. The ion volume is configured so that the inflow of the reagent gas maintains a relatively high pressure within the ion volume, thereby ensuring a density for the reagent gas that increases the probability of collisions between the incoming electrons and the molecules of the reagent gas. When electrons collide with the molecules of the reagent gas, the collisions produce ions of the reagent gas. The ions of the reagent gas then react with the analytes of the sample gas, in order to form further ions that are characteristic of the sample material. These further ions are then supplied to the mass analyzer.
It is often advantageous to collect data regarding a particular sample using both EI and CI. Although it is possible to use one mass analyzer for EI and a different mass analyzer for CI, it can be advantageous to use the same mass analyzer for both EI and CI. Due to factors such as the fact that CI and EI need to be carried out at different pressures, early attempts to switch a mass spectrometer between EI and CI involved physically removing one type of ion source from the mass analyzer and replacing it with the other type of ion source. This included venting of the vacuum chamber that contained the ion source, and then reestablishing a vacuum after the ion sources were exchanged. This approach typically took one or more hours to carry out.
Subsequently, pressure interlocks were developed that permitted one type of ion source to be removed and replaced with the other type of ion source, without breaking the vacuum. This reduced the amount of time needed to exchange the ion sources, typically to several minutes. Ideally, however, it is desirable to be able to switch between EI and CI sufficiently quickly so that, for example, either EI or CI ionization techniques can be utilized within a single chromatographic run for different analytes, or so that both EI and CI spectra can be acquired within the elution time of individual analytes.
A later-developed ion source simultaneously carries out both CI and EI. The CI and EI ion volumes are maintained at different potentials, thereby making it possible to electromagnetically select ions from either ion volume for analysis, while excluding ions from the other ion volume. While this approach has been generally adequate for its intended purposes, it has not been entirely satisfactory in all respects. As one example, this approach continuously carries out both EI and CI ionization. This results in a relatively rapid buildup of contaminants on the surfaces of both ion volumes, and the contaminants act to reduce the sensitivity of the system. Consequently, the ion source must be disassembled on a relatively frequent basis in order to clean the interior surfaces of both ion volumes. Moreover, in this configuration, the electromagnetic selection of ions presents competing considerations. On the one hand, ions from the two ion volumes have overlapping kinetic energy distributions that make it difficult to completely exclude ions from one volume in favor the other. But on the other hand, if the electromagnetic fields used for selection are increased in an attempt to improve the separation, there is the possibility of compromising sensitivity in regard to ions that are being selected.